Javascript - turn array with negative indices to normal array

objToArr.js

/**
 *  Returns a normal iterable array from an array with -ve numbers, padding any missing elements with the provided filler
 * Use case - algo problems where we move in 2d space to -ve numbered places in an arr or matrix
 * @param {Array} obj An array containing -ve numbers as indices, behaves like an object at times
 * @param {*} filler Filler material to populate missing members e.g. array or string
 * @param {Number} maxPos maximum positive value of a key in array
 * @param {Number} maxNeg max -ve value of a key in the incoming array
 * @returns {Array} Sorted array
 */
function _objToArr(obj, filler, maxPos, maxNeg) {
    // js arr with -ev nums are treated similiar to obj
    // -ve keys come after +ve numeric keys
    // +ve keys are stored in asc order 
    // -ve keys are stored in desc order
    // i.e. 0 1 2 -1 -2
    // we also need to manually iter keys since .forEach on arr ignores -ve keys

    // herein we create a left array for the -ve keys
    // right arr for +ve keys
    // fill and pad them to the max lengths then return concated result
    // using filler value for missing members
    let outArrLeft = new Array();
    let outArrRight = new Array(maxPos);
    for (let i=0; i<=maxPos; i++) {
        // fill right arr w/ +ve nums
        if (obj[i] == undefined && outArrRight[i] == undefined) {
            outArrRight[i] = filler;
        } else {
            outArrRight[i] = obj[i];
        }
    }
    for (let i=-1; i>=maxNeg; i--) {
        // fill left arr w/ -ve nums
        if (obj[i] == undefined && outArrLeft[i] == undefined) {
            outArrLeft.unshift(filler);
        } else {
            outArrLeft.unshift(obj[i]);
        }
    }
    return outArrLeft.concat(outArrRight);
}